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                                <span class="chip bg-color">高并发</span>
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                                <span class="chip bg-color">Java锁</span>
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<p>当程序中可能出现并发的情况时，就需要保证在并发情况下数据的准确性，以此确保当前用户和其他用户一起操作时，所得到的结果和他单独操作时的结果是一样的。这就叫做并发控制。并发控制的目的是保证一个用户的工作不会对另一个用户的工作产生不合理的影响。</p>
</blockquote>
<blockquote>
<p>没有做好并发控制，就可能导致脏读、幻读和不可重复读等问题。实现并发控制的主要手段分为乐观并发控制和悲观并发控制两种。无论是悲观锁还是乐观锁，都是人们定义出来的概念，可以认为是一种思想。</p>
</blockquote>
<h1 id="一、悲观锁"><a href="#一、悲观锁" class="headerlink" title="一、悲观锁"></a>一、悲观锁</h1><h2 id="1、理解"><a href="#1、理解" class="headerlink" title="1、理解"></a>1、理解</h2><p>  在高并发场景下，当要对数据库中的一条数据进行修改的时候，为了避免同时被其他人修改，最好的办法就是直接对该数据进行加锁以防止并发。</p>
<p>  为什么叫做悲观锁呢？因为这是一种对数据的修改抱有悲观态度的并发控制方式。我们一般认为数据被并发修改的概率比较大，所以每次在拿数据的时候都会上锁。</p>
<ul>
<li>以数据库锁机制实现悲观锁：悲观锁的实现，往往依靠数据库提供的锁机制，也只有数据库层提供的锁机制才能真正保证数据访问的排他性，否则，即使在本系统中实现了加锁机制，也无法保证外部系统不会修改数据。</li>
<li>使用 <strong>synchronized</strong> 关键字实现：synchronized是有序性的，一个变量在同一时刻只允许一条线程对其进行 lock 操作。</li>
</ul>
<p><img src="/blog/images/%E7%AC%94%E8%AE%B0/%E9%AB%98%E5%B9%B6%E5%8F%91%E5%9C%BA%E6%99%AF%E4%B9%90%E8%A7%82%E9%94%81%E4%B8%8E%E6%82%B2%E8%A7%82%E9%94%81/1.jpg"></p>
<h2 id="2、以数据库锁机制实现悲观锁"><a href="#2、以数据库锁机制实现悲观锁" class="headerlink" title="2、以数据库锁机制实现悲观锁"></a>2、以数据库锁机制实现悲观锁</h2><blockquote>
<p><strong>场景</strong>：有用户A和用户B，在同一家店铺去购买同一个商品，但是商品的可购买数量只有一个。在不加锁的情况下，如果用户A和用户B同时下单，就会报错。</p>
</blockquote>
<p>  传统的关系型数据库使用这种锁机制，比如行锁、表锁、读锁、写锁等，都是在操作之前先上锁（<strong>以下内容以MySQL数据库为例</strong>）。</p>
<h3 id="2-1、行锁"><a href="#2-1、行锁" class="headerlink" title="2.1、行锁"></a>2.1、行锁</h3><p>  行锁：操作时只锁某一行，不对其他行有影响。行锁偏向InnoDB存储引擎，开销大，加锁慢，会出现死锁，锁定粒度最小，发生锁冲突的概率最低，并发度也很高。InnoDB与MyISAM的最大不同有两点：一是支持事务，二是采用了行级锁。由于MySQL的行锁是针对索引加的锁，不是针对记录加的锁，所以虽然是访问不同行的记录，但是如果是使用相同的索引键，是会出现锁冲突的。</p>
<p>  创建数据库：并且设置数据库引擎为：InnoDB</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token identifier"><span class="token punctuation">`</span>test_table<span class="token punctuation">`</span></span> <span class="token punctuation">(</span>
  <span class="token identifier"><span class="token punctuation">`</span>id<span class="token punctuation">`</span></span> <span class="token keyword">INT</span><span class="token punctuation">(</span><span class="token number">11</span><span class="token punctuation">)</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span> <span class="token keyword">AUTO_INCREMENT</span><span class="token punctuation">,</span>
  <span class="token identifier"><span class="token punctuation">`</span>name<span class="token punctuation">`</span></span> <span class="token keyword">VARCHAR</span><span class="token punctuation">(</span><span class="token number">40</span><span class="token punctuation">)</span> <span class="token keyword">DEFAULT</span> <span class="token boolean">NULL</span><span class="token punctuation">,</span>
  <span class="token keyword">PRIMARY</span> <span class="token keyword">KEY</span> <span class="token punctuation">(</span><span class="token identifier"><span class="token punctuation">`</span>id<span class="token punctuation">`</span></span><span class="token punctuation">)</span>
<span class="token punctuation">)</span> <span class="token keyword">ENGINE</span><span class="token operator">=</span><span class="token keyword">InnoDB</span> <span class="token keyword">DEFAULT</span> <span class="token keyword">CHARSET</span><span class="token operator">=</span>utf8<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<ul>
<li>1：需要关闭mysql自动提交</li>
</ul>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SET</span> autocommit <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<ul>
<li>2：session1为某一行数据添加锁（FOR UPDATE）</li>
</ul>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> test_table <span class="token keyword">WHERE</span> id <span class="token operator">=</span> <span class="token number">1</span> <span class="token keyword">FOR</span> <span class="token keyword">UPDATE</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<ul>
<li>3：session2为另一行数据添加锁。虽然session2访问的是和session1不同的记录，但是因为使用了相同的索引，所以需要等待锁：</li>
</ul>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> test_table <span class="token keyword">WHERE</span> id <span class="token operator">=</span> <span class="token number">2</span> <span class="token keyword">FOR</span> <span class="token keyword">UPDATE</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<h3 id="2-2、读锁（表锁）"><a href="#2-2、读锁（表锁）" class="headerlink" title="2.2、读锁（表锁）"></a>2.2、读锁（表锁）</h3><p>  读锁（共享锁）：针对同一份数据，多个操作可以同时进行而不会相互影响。</p>
<p>  创建数据库：并且设置数据库引擎为：MyISAM</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token identifier"><span class="token punctuation">`</span>test_table<span class="token punctuation">`</span></span> <span class="token punctuation">(</span>
  <span class="token identifier"><span class="token punctuation">`</span>id<span class="token punctuation">`</span></span> <span class="token keyword">INT</span><span class="token punctuation">(</span><span class="token number">11</span><span class="token punctuation">)</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span> <span class="token keyword">AUTO_INCREMENT</span><span class="token punctuation">,</span>
  <span class="token identifier"><span class="token punctuation">`</span>name<span class="token punctuation">`</span></span> <span class="token keyword">VARCHAR</span><span class="token punctuation">(</span><span class="token number">40</span><span class="token punctuation">)</span> <span class="token keyword">DEFAULT</span> <span class="token boolean">NULL</span><span class="token punctuation">,</span>
  <span class="token keyword">PRIMARY</span> <span class="token keyword">KEY</span> <span class="token punctuation">(</span><span class="token identifier"><span class="token punctuation">`</span>id<span class="token punctuation">`</span></span><span class="token punctuation">)</span>
<span class="token punctuation">)</span> <span class="token keyword">ENGINE</span><span class="token operator">=</span>MYISAM <span class="token keyword">DEFAULT</span> <span class="token keyword">CHARSET</span><span class="token operator">=</span>utf8<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<ul>
<li>给表test_table添加读锁</li>
</ul>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">LOCK</span> <span class="token keyword">TABLE</span> <span class="token identifier"><span class="token punctuation">`</span>test_table<span class="token punctuation">`</span></span> <span class="token keyword">READ</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<ul>
<li>当有两个session用户访问同一个表test_table时，session1给test_table添加读锁时发现：</li>
<li>session1用户可以查询这张表的数据，session2也可以查询这张表的数据</li>
<li>session1用户修改这张表失败了，因为现在把这张表锁了，所以修改操作会出错</li>
</ul>
<p><img src="/blog/images/%E7%AC%94%E8%AE%B0/%E9%AB%98%E5%B9%B6%E5%8F%91%E5%9C%BA%E6%99%AF%E4%B9%90%E8%A7%82%E9%94%81%E4%B8%8E%E6%82%B2%E8%A7%82%E9%94%81/2.jpg"></p>
<ul>
<li>session1查看别的锁时，发现也失败了，因为当锁住了test_table这张表时，在对这张表操作完之后应该释放掉这把锁才能操作别的表</li>
</ul>
<p><img src="/blog/images/%E7%AC%94%E8%AE%B0/%E9%AB%98%E5%B9%B6%E5%8F%91%E5%9C%BA%E6%99%AF%E4%B9%90%E8%A7%82%E9%94%81%E4%B8%8E%E6%82%B2%E8%A7%82%E9%94%81/3.jpg"></p>
<ul>
<li>session2想要修改这张已经被锁住的test_table表的内容时，会陷入阻塞中，因为test_table的锁还有释放不能进行修改操作</li>
<li>unlock tables 解除读锁</li>
</ul>
<h3 id="2-3、写锁（表锁）"><a href="#2-3、写锁（表锁）" class="headerlink" title="2.3、写锁（表锁）"></a>2.3、写锁（表锁）</h3><p>  写锁（排他锁）：当前写操作没有完成前，他会阻断其他写锁和读锁</p>
<ul>
<li>1：首先进行加锁：</li>
</ul>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">LOCK</span> <span class="token keyword">TABLE</span> <span class="token identifier"><span class="token punctuation">`</span>test_table<span class="token punctuation">`</span></span> <span class="token keyword">WRITE</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<ul>
<li>2：进行查找表test_table里的内容，发现可以查出来</li>
<li>3：当想要select别的表和修改mylock表时，就会报错，理由和读锁是一样，要先释放才可以查找别的表</li>
<li>4：当session2想要查看被写锁锁住的表时，会陷入阻塞，只有当释放锁时才可以select出来</li>
</ul>
<h2 id="3、使用-synchronized-关键字实现"><a href="#3、使用-synchronized-关键字实现" class="headerlink" title="3、使用 synchronized 关键字实现"></a>3、使用 synchronized 关键字实现</h2><p>  synchronized有一下三种使用方式</p>
<ul>
<li>修饰实例方法，为当前实例加锁，进入同步方法前要获得当前实例的锁。</li>
<li>修饰静态方法，为当前类对象加锁，进入同步方法前要获得当前类对象的锁。</li>
<li>修饰代码块，指定加锁对象，对给定对象加锁，进入同步代码块前要获得给定对象的锁。</li>
</ul>
<p>  有一个要注意的地方是对静态方法的修饰可以和实例方法的修饰同时使用，不会阻塞，因为一个是修饰的Class类，一个是修饰的实例对象。</p>
<h1 id="二、乐观锁"><a href="#二、乐观锁" class="headerlink" title="二、乐观锁"></a>二、乐观锁</h1><h2 id="1、理解-1"><a href="#1、理解-1" class="headerlink" title="1、理解"></a>1、理解</h2><blockquote>
<p>乐观锁是相对悲观锁而言的，乐观锁假设数据一般情况不会造成冲突，操作数据时不会对操作的数据进行加锁，只有到数据提交的时候才通过一种机制来验证数据是否存在冲突。</p>
</blockquote>
<blockquote>
<p>乐观锁实现方式乐观锁不需要借助数据库的锁机制，主要就是两个步骤：冲突检测和数据更新。</p>
</blockquote>
<ul>
<li>CAS 实现：Java 中java.util.concurrent.atomic包下面的原子变量使用了乐观锁的一种 CAS 实现方式。</li>
<li>版本号控制：一般是在数据表中加上一个数据版本号 version 字段或者时间戳 timestamp，表示数据被修改的次数。当数据被修改时，version 值会 +1。当线程 A 要更新数据时，在读取数据的同时也会读取 version 值，在提交更新时，若刚才读取到的 version 值与当前数据库中的 version 值相等时才更新，否则重试更新操作，直到更新成功。</li>
</ul>
<h2 id="2、CAS实现"><a href="#2、CAS实现" class="headerlink" title="2、CAS实现"></a>2、CAS实现</h2><p>  CAS意思是：比较并替换。CAS需要有3个操作数：内存地址V，旧的预期值A，即将要更新的目标值B。</p>
<p>  CAS指令执行时，当且仅当内存地址V的值与预期值A相等时，将内存地址V的值修改为B，否则就什么都不做。整个比较并替换的操作是一个原子操作。下面以<strong>AutoInteger</strong>为例：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token annotation punctuation">@Test</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test20</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator">&lt;=</span> <span class="token number">5</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">MyThread</span> thrend <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyThread</span><span class="token punctuation">(</span><span class="token string">"线程"</span> <span class="token operator">+</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Thread</span> thread <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>thrend<span class="token punctuation">)</span><span class="token punctuation">;</span>
        thread<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">2000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">MyCount</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>


<span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">MyThread</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
    <span class="token class-name">MyThread</span><span class="token punctuation">(</span><span class="token class-name">String</span> threadName<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> threadName<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span><span class="token number">20</span><span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token class-name">MyCount</span><span class="token punctuation">.</span>count<span class="token operator">++</span><span class="token punctuation">;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">"count："</span><span class="token operator">+</span> <span class="token class-name">MyCount</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">MyCount</span> <span class="token punctuation">{</span>
    <span class="token keyword">static</span> <span class="token keyword">int</span> count <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>  以上内容为：一个共享变量count=1，开5个线程，每个线程加20次，结果一般来说都会小于100（数量大了结果一般都不一致）。原因是在出现多次同一时间多个线程拿到了同一个count，造成了数量和数值错误。</p>
<p>  现在修改一个代码，将int变成AtomicInteger，如下：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token annotation punctuation">@Test</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test20</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator">&lt;=</span> <span class="token number">5</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">MyThread</span> thrend <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyThread</span><span class="token punctuation">(</span><span class="token string">"线程"</span> <span class="token operator">+</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Thread</span> thread <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>thrend<span class="token punctuation">)</span><span class="token punctuation">;</span>
        thread<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">2000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">MyCount</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>


<span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">MyThread</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
    <span class="token class-name">MyThread</span><span class="token punctuation">(</span><span class="token class-name">String</span> threadName<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> threadName<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span><span class="token number">20</span><span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token comment">// 加1方法</span>
            <span class="token class-name">MyCount</span><span class="token punctuation">.</span>count<span class="token punctuation">.</span><span class="token function">getAndIncrement</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">"count："</span><span class="token operator">+</span> <span class="token class-name">MyCount</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">MyCount</span> <span class="token punctuation">{</span>
    <span class="token keyword">static</span> <span class="token class-name">AtomicInteger</span> count <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">AtomicInteger</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>结果显示：每次结果都是100，AtomicInteger加1方法的源码如下：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">getAndIncrement</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 第一个参数当前对象地址，第二个参数数据偏移量，第三个参数每次指定默认加1</span>
    <span class="token keyword">return</span> unsafe<span class="token punctuation">.</span><span class="token function">getAndAddInt</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> valueOffset<span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">// 这个方法使用的就是CAS,核心在于循环比较内存里面的值和当前值是否相等，如果相等就用新值覆盖</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">getAndAddInt</span><span class="token punctuation">(</span><span class="token class-name">Object</span> var1<span class="token punctuation">,</span> <span class="token keyword">long</span> var2<span class="token punctuation">,</span> <span class="token keyword">int</span> var4<span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">int</span> var5<span class="token punctuation">;</span>   
    <span class="token keyword">do</span> <span class="token punctuation">{</span>
        <span class="token comment">//如果a，b线程同时执行这个方法，a线程拿到值1后cpu执行时间到了挂起，b开始执行，也拿到1，但是没有挂起，接着将值变成了2</span>
        var5 <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">getIntVolatile</span><span class="token punctuation">(</span>var1<span class="token punctuation">,</span> var2<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token comment">//这个时候a线程恢复执行，去比较的时候发现手上的1 和内存里面的值2不等，这个时候他要进行下一个循环，占用cpu</span>
    <span class="token punctuation">}</span> <span class="token keyword">while</span><span class="token punctuation">(</span><span class="token operator">!</span><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">compareAndSwapInt</span><span class="token punctuation">(</span>var1<span class="token punctuation">,</span> var2<span class="token punctuation">,</span> var5<span class="token punctuation">,</span> var5 <span class="token operator">+</span> var4<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token keyword">return</span> var5<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>除了AtomicInteger，还有AtomicLong、AtomicBoolean…..都在java.util.current.atomic包下面，采用了CAS机制来实现加锁。但是CAS容易引发ABA问题：</p>
<ul>
<li>比如说线程一从数据库中取出库存数 3，这时候线程二也从数据库中取出库存数 3，并且线程二进行了一些操作变成了 2。</li>
<li>然后线程二又将库存数变成 3，这时候线程一进行 CAS 操作发现数据库中仍然是 3，然后线程一操作成功。</li>
<li>尽管线程一的 CAS 操作成功，但是不代表这个过程就是没有问题的。</li>
</ul>
<p>CAS机制类似于以下SQL：</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment"># 查询出商品库存 quantity=3</span>
<span class="token keyword">select</span> quantity <span class="token keyword">from</span> goods <span class="token keyword">where</span> id <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token comment"># 修改商品库存为2</span>
<span class="token keyword">update</span> goods <span class="token keyword">set</span> quantity<span class="token operator">=</span>quantity<span class="token operator">-</span><span class="token number">1</span> <span class="token keyword">where</span> id<span class="token operator">=</span><span class="token number">1</span> <span class="token operator">and</span> quantity<span class="token operator">=</span><span class="token number">3</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="3、版本控制实现"><a href="#3、版本控制实现" class="headerlink" title="3、版本控制实现"></a>3、版本控制实现</h2><blockquote>
<p>用版本控制实现乐观锁可以解决CAS可能造成的ABA问题。乐观锁每次在执行数据修改操作时，都会带上一个版本号，一旦版本号和数据的版本号一致就可以执行修改操作并对版本号执行 +1 操作，否则就执行失败。因为每次操作的版本号都会随之增加，所以不会出现 ABA 问题。除了 version 以外，还可以使用时间戳，因为时间戳天然具有顺序递增性。</p>
</blockquote>
<p>比如以下SQL：</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment"># 查询出商品库存和该数据的版本号 quantity=3 version=1</span>
<span class="token keyword">select</span> quantity<span class="token punctuation">,</span>version <span class="token keyword">from</span> goods <span class="token keyword">where</span> id <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token comment"># 修改商品库存为2</span>
<span class="token keyword">update</span> goods <span class="token keyword">set</span> quantity<span class="token operator">=</span>quantity<span class="token operator">-</span><span class="token number">1</span> <span class="token punctuation">,</span>version<span class="token operator">=</span>version<span class="token operator">+</span><span class="token number">1</span> <span class="token keyword">where</span> id<span class="token operator">=</span><span class="token number">1</span> <span class="token operator">and</span> version<span class="token operator">=</span><span class="token number">1</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>以上 SQL 其实还是有一定的问题的，就是一旦遇上高并发的时候，就只有一个线程可以修改成功，那么就会存在大量的失败，总让用户感知到失败显然是不合理的。所以，还是要想办法减少乐观锁的粒度。一个比较好的建议，就是减小乐观锁力度，最大程度的提升吞吐率，提高并发能力。如下：</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment"># 查询出商品库存和该数据的版本号 quantity=3</span>
<span class="token keyword">select</span> quantity<span class="token punctuation">,</span>version <span class="token keyword">from</span> goods <span class="token keyword">where</span> id <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token comment"># 修改商品库存为2</span>
<span class="token keyword">update</span> goods <span class="token keyword">set</span> quantity<span class="token operator">=</span>quantity<span class="token operator">-</span><span class="token number">1</span> <span class="token keyword">where</span> id<span class="token operator">=</span><span class="token number">1</span> <span class="token operator">and</span> quantity<span class="token operator">-</span><span class="token number">1</span><span class="token operator">&gt;</span><span class="token number">0</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>



                
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